The present invention relates to a stud for stud welding, having a shank, having an axially adjoining flange which projects radially relative to the shank, and having an annular section which axially adjoins the flange and whose front, radially extending annular surface is designed as a welding surface which is to be welded to a workpiece.
The invention also relates to a fastening arrangement having a workpiece and a stud welded to the workpiece and also relates to a method of welding a stud to a workpiece and to the use of such a stud for welding onto a workpiece.
The invention generally concerns the field of stud welding. This welding process is used, inter alia, to a great extent in the field of vehicle technology. In this case, studs are welded to body sheets and serve as anchors for plastic fastening elements (clips), dashboards, etc.
The trend in recent years is to reduce the thickness of the body sheets in order to reduce, for example, the weight of the vehicle. The welding of studs onto body sheets that are becoming increasingly thinner turns out to be difficult. As a rule, the studs have a conically tapering welding surface. In order to fuse this surface during the stud welding, a relatively high input of energy is required. This may lead to “burn-through” of the body sheet.
Consequently, a welding stud which has a concave recess and is designed for capacitor discharge welding has been proposed in document U.S. Pat. No. 3,671,710. In this case, the recess together with the outside diameter of the stud results in an annular flange. This is intended to improve the uniformity of the molten pool.
Document DE 196 11 711 A1 discloses a welding element having an annular flange structure. The annular flange has a margin narrowing in cross section, such that an approximately linear edge is obtained at the front end of the stud. The welding edge is set down on the workpiece. The stud is then pulled back for forming an arc. Finally, the stud is pushed into the complete molten pool. An approximately annular welding spot results overall, and a cavity is formed between the stud and the workpiece in the interior of this annular welding spot. It is mentioned that the configuration of the stud is intended to permit welding of the stud to relatively thin sheets by means of the drawn-arc process.
Finally, document WO 2004/033923 A1 discloses a fastening element in which a stud is welded onto a sheet (in particular a sandwich sheet). The stud has a region which serves as a predetermined breaking point in order to achieve a defined failure case. A nut is screwed onto the stud, and the strength of the stud is adapted to the strength of the nut, such that the nut breaks first before the stud breaks. Provided the nut does not break, the stud breaks before the stud is torn out of the sheet.
In other words, a torque is not applied to the stud until during the assembly when a plastic fastening element or the like is actually put onto the stud. If a defect affecting the stud and/or the sheet occurs at this moment for certain reasons, relatively expensive rework is necessary. For at this moment the sheet is already painted as a rule and the repair of the welding spot and the re-application of a stud consequently require complicated subsequent treatment of the surface of the body sheet.
The stud used has a shank, a flange section and an axially adjoining annular section which together form a head section. The ratio of axial length of the annular section to axial length of the head section (which is formed by the flange and the annular section) is to be less than 50%, in particular within the range of 25% to 35%. Considerable importance is also attached to other dimensions or ratios of dimensions of the head section in this application. The outside diameter of the head section is designed to be round in this case.
Against the above background, an object of the invention is to specify a cost-effective stud, an improved fastening arrangement and an improved method of welding a stud onto a workpiece, wherein the workpiece has a small thickness and wherein a situation in which expensive rework has to be carried out in the assembly area in the event of failure of a welded joint can be avoided.